1. Field of the Invention
The present invention relates to a soldering method and a soldering apparatus which are used for bonding parts or equipment components. In particular, the present invention relates to a soldering method and a soldering apparatus which are applicable to manufacture of circuit boards which mount an electronic part, printed circuits and the like, and which are suitable for soldering treatment with use of a solder which contains no lead.
2. Related Art
Soldering is an art for bonding objects to each other by using a substance having a low melting point, and it has been used since old times. It is generally said that the origin of the soldering can be traced up to ancient Mesopotamian civilization. In current industries, soldering is widely used in bonding electronic devices, for example, bonding such electronic devices as semiconductor, microprocessor, memory and resistor to a substrate. Its advantage is not only to fix a part to the substrate, but also to form electrical connection by electric conductivity of the metal contained in the solder. This point is different from organic series adhesive agents.
The solder which is generally used is eutectic solder composed of tin and lead, having a eutectic point of 183.degree. C. This is used for bonding sheet materials of copper or the like. It is characterized by the eutectic point which is not only lower than the melting point of metallic base material to be soldered, but also lower than a temperature in which gasification of thermosetting resin begins. Further, it has been known that the tin component of the eutectic solder forms a particular intermetallic compound on an interface with a copper plate, thereby intensifying bonding strength between the solder and copper. In addition to the eutectic solder composed of tin and lead having such a characteristic, solder composed of tin and zinc, solder composed of silver and tin, etc. have been used on trial. However, their wettabilities are poor, thereby providing a poor connectability. Thus they have not been conventionally used in actual fields.
As described above, bonding by soldering is still important in manufacture of electronic devices. In today's world in which personal electronic devices such as a personal computer, a cellular phone, a pager and the like have been spreading quickly, the importance of solder in electronic device mounting technology has been intensified.
Spreading of electronic devices contributes to enrichment of people's lives. However, on the contrary, if a large amount of electronic devices disused are scrapped, there is a fear that wasted electronic devices may pollute the environment. Therefore, recycling of disused articles and elimination of harmful substances from the materials to be used for manufacture of articles have been advocated. Especially, the elimination of harmful substances is desired in view of prevention of environmental polution.
From such a condition, bonding skill by using solder containing no lead has been demanded. However, solder in which lead is substituted by other metal or solder containing a combination of other metals cannot be handled at such a low temperature that bad influence upon the base metal by high temperatures can be avoided, and the wettability is so poor that the solder is not fixed to the base metal satisfactorily. Thus, such a solder can be applied to neither fine soldering treatment such as mounting in semiconductor devices not ordinary bonding by solder. Particularly, solder with tin and zinc has too many problems to be solved, therefore, it has been considered impossible to use it for actual application in electronic mounting (ref. Tadashi TAKEMOTO et al., "Environmental Conscious Lead-free Solder Alloys compatible with Fine Pitch Soldering", Proceedings of The Third International Conference on Ecomaterials (Sep. 10-12, 1997, Tsukuba, Japan)).
To enable use of solder without lead in fine soldering works such as thick film formation, conductor circuit formation and semiconductor mounting, a screen printing method using solder paste in which solder powder and flux are mixed has been proposed. The flux used in solder paste is generally classified to organic compound, inorganic compound and resin. In the case when organic compound or resin is used, halogen salt, organic acid salt and the like of organic acid and amino group are frequently added as active ingredient. In the case of inorganic compound, ammonium halide, zinc halide, tin halide, phosphoric acid, hydroacid halide or the like are often added. Since these additives corrode metals, inspection for corrosion due to flux residue after reflow of solder paste is necessary. Moreover, organic substance evaporating when paste is heated to remove flux, must be treated.